On-demand all-wheel drive system

ABSTRACT

A drive axle assembly for use in an all-wheel drive vehicle having a first hydraulic coupling operable to automatically transfer drive torque to a secondary driveline in response to slip of the primary driveline and a second hydraulic coupling operable to bias torque and limit slip between the wheels of the secondary driveline.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication Serial No. 60/280,797, filed Apr. 2, 2001.

FIELD OF THE INVENTION

[0002] The present invention relates generally to hydraulic couplingsfor use in motor vehicle driveline applications for limiting slip andtransferring torque between rotary members. More specifically, a driveaxle assembly for an all-wheel drive vehicle is disclosed having a pairof hydraulic couplings each having a fluid pump, a multi-plate clutchassembly, and a fluid distribution system operable to control actuationof the clutch assembly.

BACKGROUND OF THE INVENTION

[0003] In all-wheel drive vehicles, it is common to have a secondarydrive axle that automatically receives drive torque from the drivetrainin response to lost traction at the primary drive axle. In suchsecondary drive axles, it is known to provide a pair of clutchassemblies connecting each axleshaft to a prop shaft that is driven bythe drivetrain. For example, U.S. Pat. No. 4,650,028 discloses asecondary drive axle equipped with a pair of viscous couplings. Inaddition, U.S. Pat. Nos. 5,964,126, 6,095,939 and 6,155,947 eachdisclose secondary drive axles equipped with a pair of pump-actuatedmulti-plate clutch assemblies. In contrast to these passively-controlledsecondary drive axles, U.S. Pat. No. 5,699,888 teaches of a secondarydrive axle having a pair of multi-plate clutches that are actuated byelectromagnetic actuators that are controlled by an electronic controlsystem.

SUMMARY OF THE INVENTION

[0004] An object of the present invention is to provide a drive axleassembly equipped with a pair of hydraulic couplings which are operablyarranged for coupling a vehicle drivetrain to a pair of axleshafts.

[0005] It is another object of the present invention to provide a driveaxle assembly for use in an all-wheel drive vehicle having a firsthydraulic coupling operable to automatically transfer drive torque to asecondary driveline in response to slip of the primary driveline and asecond hydraulic coupling operable to bias torque and limit slip betweenthe wheels of the secondary driveline.

[0006] In carrying out the above object, the drive axle assembly of thepresent invention includes a pinion shaft, a first hydraulic couplingoperably disposed between a driven prop shaft and the pinion shaft, anda differential drive module. The differential drive module includes adrive case driven by the pinion shaft, a differential unit operablyinterconnecting the drive case to a pair of axleshafts, and a secondhydraulic coupling operably disposed between the drive case and one ofthe axleshafts.

[0007] The first hydraulic coupling includes a multi-plate clutchassembly and a clutch actuator. The clutch actuator includes a fluidpump and a piston assembly. The fluid pump is operable for pumping fluidin response to a speed differential between the pinion shaft and theprop shaft. The piston assembly includes a piston retained for slidingmovement in a piston chamber and a multi-function control valve. Thepump supplies fluid to the piston chamber such that a clutch engagementforce exerted by the piston on the multi-plate clutch assembly isproportional to the fluid pressure in the piston chamber. The controlvalve is mounted to the piston and provides a pressure relief functionfor setting a maximum fluid pressure within the piston chamber. Thecontrol valve also provides a thermal unload function for releasing thefluid pressure within the piston chamber when the fluid temperatureexceeds a predetermined temperature value.

[0008] In accordance with an optional construction, the multi-functioncontrol valve of the present invention can also provide a flow controlfunction for regulating the fluid pressure in the piston chamber. Theflow control function can further include a thermal compensation featurefor accommodating viscosity variations in the fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Further objects, features and advantages of the present inventionwill become readily apparent from the following detailed specificationand the appended claims which, in conjunction with the drawings, setforth the best mode now contemplated for carrying out the invention.Referring to the drawings:

[0010]FIG. 1 is a schematic view of a motor vehicle drivetrain equippedwith a secondary drive axle assembly constructed in accordance with thepresent invention;

[0011]FIG. 2 is a sectional view of the secondary drive axle assembly ofthe present invention;

[0012]FIG. 3 is a sectional view of an on-demand hydraulic couplingassociated with the secondary drive axle assembly;

[0013]FIG. 4 is an enlarged partial view taken from FIG. 3 showingcomponents of the hydraulic coupling in greater detail;

[0014]FIG. 5 is a schematic diagram illustrating a hydraulic controlcircuit associated with the on-demand hydraulic coupling shown in FIG.3;

[0015]FIG. 6 is a sectional view of a differential drive moduleassociated with the secondary drive axle of the present invention;

[0016]FIGS. 7 through 10 are various exploded and sectional perspectiveviews of a slightly modified version of the on-demand hydrauliccoupling; and

[0017]FIG. 11 is a modified drive module adapted for use with thesecondary drive axle assembly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] In general, the present invention is directed to ahydromechanical limited slip and torque transfer device, hereinafterreferred to as a drive axle assembly, for use in connecting thedrivetrain to a pair of axleshafts associated with a secondary drivelineof an all-wheel drive vehicle. However, the drive axle assembly can alsofind application in other driveline applications including, but notlimited to, limited slip differentials of the type used in full-timetransfer cases and front-wheel drive transaxles. Furthermore, thisinvention advances the technology in the field of hydraulically-actuatedcouplings of the type requiring pressure relief and thermal unloading toprevent damage to the driveline components.

[0019] With reference to FIG. 1, a schematic layout for a vehiculardrivetrain 10 is shown to include a powertrain 12 driving a first orprimary driveline 14 and a second or secondary driveline 16. Powertrain12 includes an engine 18 and a transaxle 20 arranged to provide motivepower (i.e., drive torque) to a pair of wheels 22 associated withprimary driveline 14. Primary driveline 14 further includes a pair ofhalfshafts 24 connecting wheels 22 to a differential assembly (notshown) associated with transaxle 20. Secondary driveline 16 includes apower take-off unit (PTU) 26 driven by transaxle 20, a prop shaft 28driven by PTU 26, a pair of axleshafts 30 connected to a pair of wheels32, and a drive axle assembly 34 operable to transfer drive torque frompropshaft 28 to one or both axleshafts 30.

[0020] Referring to FIGS. 2 through 7, the components associated withdrive axle assembly 34 will be now detailed. Drive axle assembly 34includes a multi-piece housing 40, an input shaft 42, a first hydrauliccoupling 44, and a rear differential module 46. Input shaft 42 isrotatably supported in housing 40 by a bearing assembly 48 and sealedrelative thereto via a seal assembly 50. A yoke 52 is secured to inputshaft 42 and is adapted for connection to propshaft 28. Drive module 46includes a pinion shaft 54, a bevel-type differential gearset 56, a pairof output shafts 58 and 60 adapted for connection to axleshafts 30, anda second hydraulic coupling 62. In operation, first hydraulic coupling44 is operable to transfer drive torque from input shaft 42 to pinionshaft 54 in response to excessive interaxle speed differentiationbetween propshaft 28 and differential gearset 56. Second hydrauliccoupling 62 is operable to limit intraaxle slip in response to excessivespeed differentiation between output shafts 58 and 60.

[0021] First hydraulic coupling 44 includes a transfer clutch 70 and aclutch actuator 72. Transfer clutch 70 is a multi-plate clutch assemblyincluding a clutch hub 74 fixed (i.e., splined) to pinion shaft 54 and aclutch pack 76 of interleaved inner and outer clutch plates that arerespectively splined to hub 74 and a clutch drum 78. Clutch actuator 72includes a fluid pump 90 disposed in a pump chamber formed between andend plate 92 and a piston housing 94, and a piston assembly 96 retainedin an annular piston chamber 98 formed in piston housing 94. Clutch drum78 is fixed (i.e., welded) to piston housing 94. As seen, a bearingassembly 102 supports end plate 92 for rotation relative to input shaft42.

[0022] Piston assembly 96 is supported for axial sliding movement inpiston chamber 98 for applying a compressive clutch engagement force onclutch pack 76, thereby transferring drive torque and limiting relativerotation between input shaft 42 and pinion shaft 54. The amount oftorque transferred is progressive and is proportional to the magnitudeof the clutch engagement force exerted by piston assembly 96 on clutchpack 76 which, in turn, is a function of the fluid pressure withinpiston chamber 98. Moreover, the fluid pressure generated by pump 90 anddelivered to piston chamber 98 is largely a function of the speeddifferential between propshaft 28 and pinion shaft 54.

[0023] With particular reference to FIG. 5, a fluid distribution andvalving arrangement is shown for controlling the delivery of fluid topiston chamber 98. The fluid distribution system includes a first flowpath 104 for supplying hydraulic fluid from a sump 106 to an inletreservoir 108 located at the inlet or suction side of fluid pump 90, anda second flow path 110 for supplying fluid from the discharge or outletside of pump 90 to piston chamber 98. A third flow path 112 extendsthrough piston assembly 96 for venting fluid from piston chamber 98 intoa clutch chamber 114 in close proximity to clutch pack 76. Amulti-function control valve 116 forms part of piston assembly 96 andprovides at least two functional modes of operation. The first mode,hereinafter referred to as its pressure relief function, isschematically illustrated by a pressure relief valve 118. The secondmode of operation, hereinafter referred to as its thermal unloadfunction, is schematically indicated by a thermal unload valve 120. Witheach function, fluid discharged from piston chamber 98 is delivered toclutch chamber 114 for cooling clutch pack 76 and is then returned tosump 106 via a fourth flow path 122. According to the structure shown,lubrication slots 124 formed in hub 74 and exhaust ports 126 formed indrum 78 define fourth flow path 122.

[0024] First flow path 104 is defined by a pair of inlet ports 130formed through first end plate 92. A one-way check valve 132 is providedfor selectively opening and closing each of inlet ports 130.Specifically, one-way check valves move between “open” and “closed”positions in response to the direction of pumping action generated byfluid pump 90. Rotation of the pump components in a first direction actsto open one of check valves 132 and to close the other for permittingfluid to be drawn from sump 106 into inlet reservoir 108. The oppositeoccurs in the case of pumping in the reverse rotary direction, therebyassuring bi-directional operation of pump 90. Check valves 132 arepreferably reed-type valves mounted on rivets secured to end plate 92.Check valves 132 are of the normally-closed type to maintain fluidwithin inlet reservoir 108.

[0025] A valving arrangement associated with second flow path 110includes a second pair of one-way check valves 134 that are located in apair of flow passages 136 formed in piston housing 94 between the outletof pump 88 and piston chamber 98. As before, the direction of pumpingaction establishes which of check valves 134 is in its “open” positionand which is in its “closed” position to deliver pump pressure to pistonchamber 98. Upon cessation of pumping action, both check valves 134return to their closed position to maintain fluid pressure in pistonchamber 98. Thus, check valves 134 are also of the normally-closedvariety.

[0026] As noted, fluid pump 90 is operable for pumping hydraulic fluidinto piston chamber 98 to actuate transfer clutch 70. Fluid pump 90 isbi-directional and is capable of pumping fluid at a rate proportional tospeed differential between its pump components. In this regard, pump 90is shown as a gerotor pump assembly having a pump ring 152 that is fixed(i.e., keyed or splined) to pinion shaft 54, and an eccentric statorring 154 that is retained in an eccentric chamber formed in end plate92. Pump ring 152 has a plurality of external lobes that rotateconcentrically relative to pinion shaft 54 about a common rotationalaxis. Stator ring 154 includes a plurality of internal lobes and has anouter circumferential edge surface that is journally supported within acircular internal bore formed in end plate 92. The internal bore isoffset from the rotational axis such that, due to meshing of internallobes of stator ring 154 with external lobes of pump ring 152, relativerotation between pump ring 152 and eccentric stator ring 154 causeseccentric rotation of stator ring 154. It will be understood that fluidpump 98 can be any type of mechanical pump capable of generating pumpingaction due to a speed differential.

[0027] Piston assembly 96 is shown to include a piston 158 and controlvalve 116. Piston 158 includes a radial web segment 160 sealed by sealring 162 for movement relative to piston housing 94. Piston 158 furtherincludes one or more circumferential rim segments 166 extending from websegment 160 and which engages clutch pack 76. Piston 158 further definesa cup segment 168 within which control valve 116 is retained. Seal rings170 are provided to seal control valve 116 relative to cup segment 168and a circlip 172 is provided to retain control valve 116 in cup segment168. Control valve 116 includes a tubular housing 174 defining a seriesof inlet ports 176 and a valve chamber 178 having a series of outletports 180. Inlet ports 176 and valve chamber 178 are delineated by a rimsection 182 having a central valve aperture formed therethrough. Athermal actuator 186 is retained in pressure chamber 188 of housing 174and includes a post segment 190. A head segment of a valve member 192 isseated against the valve aperture and engages the terminal end of postsegment 190. A spring 196 mounted between an end cap 198 and valvemember 192 is operable to bias valve member 192 against the seat surfacedefined by the valve aperture for normally preventing fluid flow frominlet ports 176 to outlet ports 180. Control valve 116 is arranged suchthat inlet ports 176 communicate with piston chamber 98 with valvemember 192 directly exposed to the fluid pressure in piston chamber 98.

[0028] Hydraulic coupling 72 includes a flow regulator 200 which isoperable for setting the predetermined minimum pressure level withinpiston chamber 98 at which transfer clutch 70 is initially actuated andwhich is further operable to compensate for temperature gradients causedduring heating of the hydraulic fluid. Preferably, flow regulator 200 isa reed-type valve member secured to piston assembly 96 such that itsterminal end is normally maintained in an “open” position displaced froma by-pass port 202 formed through piston 158 for permitting by-pass flowfrom piston chamber 94 to clutch chamber 114. During low-speed relativerotation, the pumping action of fluid pump 90 causes fluid to bedischarged from piston chamber 94 through the by-pass port into clutchchamber 114. Flow regulator 200 is preferably a bimetallic valve elementmade of a laminated pair of dissimilar metallic strips having differentthermal coefficients of expansion. As such, the terminal end of thevalve element moves relative to its corresponding by-pass portregardless of changes in the viscosity of the hydraulic fluid caused bytemperature changes. This thermal compensation feature can be providedby one or more bimetallic valves. However, once the fluid in pistonchamber 98 reaching its predetermined pressure level, the terminal endof the bimetallic valve element will move to a “closed” position forinhibiting fluid flow through the by-pass port. This flow restrictioncauses a substantial increase in the fluid pressure within pistonchamber 98 which, in turn, causes piston 158 to move and exert a largeengagement force on clutch pack 86. A bleed slot (not shown) is formedin one of the by-pass port or bimetallic valve element and permits asmall amount of bleed flow even when the flow regulator is in its closedposition for gradually disengaging transfer clutch 70 when fluid pump 90is inactive.

[0029] The pressure relief function of control valve 116 occurs when thefluid pressure in piston chamber 98 is greater than that required toclose bimetallic flow regulator 200 but less than a predeterminedmaximum value. In this pressure range, the bias of spring 196 isadequate to maintain valve member 192 seated against the aperture suchthat fluid is prevented from flowing from piston chamber 94 throughoutlet ports 180. However, when the fluid pressure in piston chamber 98exceeds this maximum value, valve member 192 is forced to move inopposition to the biasing of spring 196. As such, fluid in pistonchamber 98 is permitted to flow through the aperture into valve chamber178 from where it is discharged from outlet ports 180. The fluiddischarged from outlet ports 180 circulates in clutch chamber 114 tocool clutch pack 76 and is directed to flow across actuator section 210of thermal actuator 186 prior to discharge to pump through exhaust ports126 in drum 78. Use of this pressure relief function torque limitshydraulic coupling 44 and prevents damage thereto.

[0030] The thermal unload function is actuated when the fluidtemperature detected by actuator section 210 of thermal actuator 186exceeds a predetermined maximum value. In such an instance, post segment190 moves from its retracted position shown to an extended position forcausing valve member 192 to move away from seated engagement againstaperture (or maintain valve member 192 in its displaced position duringpressure relief) and permit fluid in pressure chamber 98 to vent intoclutch chamber 114, thereby disengaging transfer clutch 70. Once pistonchamber 98 has been unloaded, the fluid and thermal actuator 186 willeventually cool to a temperature below the predetermined value, wherebypost segment 190 will return to its retracted position for resetting thethermal unload function. Thermal actuator 186 is of a type manufacturedby Therm-Omega Tech of Warminster, Pa. or Standard-Thomson of Waltham,Mass.

[0031] Referring primarily now to FIG. 6, the components of drive module46 will be described. A drive pinion 220 is formed at the end of pinionshaft 54 and is meshed with a bevel ring gear 222 fixed via bolts 224 toa drive casing 226. An end cap 228 is also fixed via bolts 224 to drivecasing 226 and is supported for rotation relative to housing 40 via abearing assembly 230. A second end cap 232 is formed at the opposite endof drive casing 226 and is rotatably supported on housing 40 via abearing assembly 234. Bevel gearset 56 includes a pair of pinion gears236 rotatably supported on opposite ends of pinion shaft 238 that isnon-rotatably fixed to drive casing 226 via a retainer screw 240.Gearset 56 further includes a first side gear 242 splined for rotationwith first output shaft 58 and a second side gear 244 splined forrotation with second output shaft 60.

[0032] Second hydraulic clutch 62 includes a biasing clutch 246 and aclutch actuator 248. Biasing clutch 246 is a multi-plate clutch assemblyhaving a clutch pack 250 of alternately interleaved inner and outerclutch plates that are respectively splined to a clutch hub 252 anddrive casing 226. Hub 252 is splined to an axial hub section 254 offirst side gear 242. Clutch actuator 248 includes a fluid pump 256 and apiston assembly 258. Pump 256 is a gerotor pump assembly disposed in apump chamber formed between end cap 228 and a piston housing 260. Aneccentric outer ring 262 of gerotor pump 256 and piston housing 260 arefixed for rotation with drive casing 226 via bolts 264. Piston assembly258 is disposed in a piston chamber 266 formed in piston housing 260. Ina preferred construction, piston assembly 258 is similar in structureand function to that of piston assembly 96 such that a control valve(not shown) similar to control valve 116 is used. As seen, seal rings270 and 272 seal a piston 274 of piston assembly 258 relative to pistonhousing 260. Assuming that piston assembly 258 is similar to pistonassembly 96, the hydraulic circuit shown in FIG. 5 would be applicableto illustrate the operation of second hydraulic coupling 62.

[0033] Pump 256 includes a pump ring 280 splined to first output shaft68, and a stator ring 282 disposed between pump ring 280 and eccentricring 262. The external lobes of pump ring 280 mesh with the internallobes of stator ring 282, with stator ring 282 journalled in aneccentric aperture formed in eccentric rig 262. Relative rotationbetween drive casing 226 and first output shaft 58 generates a fluidpumping action. Check valves 132 are retained in inlet ports formed inend cap 228 while one-way check valves 134 are retained in flow passagesformed in piston housing 260 between the outlet of pump 256 and pistonchamber 266. A pressure regulator valve is mounted in a by-pass passagethrough piston 274 to control pressurization of piston chamber 266 so asto allow a limited amount of unrestrained inter-wheel speeddifferentiation, such as during turns.

[0034] This arrangement of an in-line hydraulic coupling between propshaft 78 and pinion shaft 54 permits “on-demand” transfer of drivetorque to secondary driveline 16. Thus, all-wheel drive traction controlis provided when needed in response to a loss of traction between thefront and rear drivelines. Combining the in-line coupling with secondhydraulic coupling 62 in drive module 46 provides “front-to-back” and“side-to-side” traction control that is well suited for use inconjunction with a secondary driveline system.

[0035] Referring now to FIGS. 7 through 10, a modified version of firsthydraulic coupling, identified by reference numeral 44′, is shown.Hydraulic coupling 44′ is generally similar in structure and function tohydraulic coupling 44, with the exception that piston assembly 96′ isnow splined to drum 78′. However, the pump valving, operation of thebimetallic flow control valve and control valve 116 are substantiallysimilar.

[0036] Referring now to FIG. 11, a dual-clutch drive module 46′ is shownwhich can be used in substitution for drive module 46. Drive module 46′includes a drive case 300 to which ring gear 222 is bolted, a firsthydraulic clutch 302 connected between drive case 300 and first outputshaft 58, and a second hydraulic clutch 304 connected between drive case300 and second output shaft 60. Clutches 302 and 304 are generallysimilar to hydraulic coupling 44 and include clutch packs 76A and 76B,hydraulic pumps 90A and 90B, and piston assemblies 96A and 96B. Pumpsare located between piston housings 94A and 94B and end caps 92A and92B. Clutch 302 provides speed and torque control between drive case 300and output shaft 58 while clutch 304 provides similar control betweendrive case 300 and output shaft 60. Thus, left-to-right (i.e.,side-to-side) torque control and speed differentiation is provided.

What is claimed is:
 1. A drive axle assembly for use in a motor vehicleto transfer drive torque from a powertrain to a pair of wheels,comprising: a first hydromechanical coupling having an input shaftdriven by the powertrain, a pinion shaft, a first transfer clutchopearbly disposed between said input shaft and said pinion shaft, afirst piston disposed in a first piston chamber and actuatable to engagesaid first transfer clutch for transferring drive torque to said pinionshaft, a first fluid pump for pumping hydraulic fluid from a sump tosaid first piston chamber in response to relative rotation between saidinput shaft and said pinion shaft, and a control valve mounted to saidfirst piston, said control valve operable to vent fluid from said firstpiston chamber to said sump in response to the occurrence of either ofan over-pressure and an over-temperature condition; first and secondoutput shafts adapted for connection to the pair of wheels; adifferential assembly including a casing driven by said pinion shaft,and a gearset interconnecting said casing to said first and secondoutput shafts; and a second hydromechanical coupling having a secondtransfer clutch operably disposed between said casing and said gearsetof said differential assembly, a second piston disposed in a secondpiston chamber which is actuatable in response to fluid pressure in saidsecond piston chamber to engage said second transfer clutch for biasingtorque and limiting slip between said first and second output shafts,and a second pump for pumping fluid from said sump to said second pistonchamber in response to relative rotation between said casing and one ofsaid first and second output shafts.
 2. The drive axle of claim 1wherein said first hydromechanical coupling includes a first flow pathfor supplying hydraulic fluid from said sump to an inlet of said firstpump, a second flow path for supply hydraulic fluid from an outlet ofsaid first pump to said first piston chamber, and a third flow paththrough said first piston for venting fluid from said first pistonchamber to said sump, and wherein said control valve is located in saidthird flow path.
 3. The drive axle of claim 1 wherein said control valveincludes a valve housing defining a valve chamber in fluid communicationwith said sump and a flow port providing fluid communication betweensaid first piston chamber and said valve chamber, and a thermal unloadvalve having a thermal actuator mounted in said valve chamber and avalve member movable from a first position to a second position when thetemperature of the fluid in said valve chamber exceeds a predeterminedtemperature value, said valve member is operable in its first positionto prevent fluid flow through said flow port and is further operable inits second position to permit fluid flow through said flow port forventing said first piston chamber.
 4. The drive axle of claim 1 whereinsaid control valve includes a pressure relief valve for venting fluidfrom said first piston chamber to said sump when the fluid pressure insaid first piston chamber exceeds a predetermined pressure value.
 5. Thedrive axle of claim 1 wherein said control valve further includes a flowregulator for regulating flow of hydraulic fluid from said first pistonchamber to said sump to control the fluid pressure in said first pistonchamber for actuating said first piston.
 6. The drive axle of claim 1wherein said first piston includes a radial web segment sealed relativeto said first piston chamber, a rim segment engageable with said firsttransfer clutch, and a cup segment formed between said web segment andsaid rim segment, said control valve being mounted in said cup segment.7. The drive axle of claim 6 wherein said control valve comprises: atubular housing retained in said cup segment of said first piston anddefining an inlet port communicating with said first piston chamber, afirst chamber communicating with said inlet port, a second chambercommunicating with said sump, and an aperture formed between said firstand second chambers; and a valve member moveable from a first positionclosing said aperture when the fluid pressure in said first chamber isless than a predetermined pressure value to a second position foropening said aperture when the fluid pressure in said first chamberexceeds said predetermined pressure value.
 8. The drive axle of claim 7wherein said control valve further includes a thermal actuator operableto move said valve member from its first position to its second positionwhen the fluid temperature in said first chamber exceeds a predeterminedtemperature value.
 9. The drive axle of claim 1 wherein said gearset ofsaid differential assembly includes a pair of pinion gears supported forrotation with said casing, a first side gear coupled to said firstoutput shaft that is meshed with said pinion gears, and a second sidegear coupled to said second output shaft that is meshed with said piniongears.
 10. The drive axle of claim 9 wherein said second transfer clutchof said second hydromechanical coupling is operably disposed betweensaid casing and said second side gear.
 11. The drive axle of claim 1wherein said second hydromechanical coupling further includes a secondcontrol valve that is operable to vent fluid from said second pistonchamber to said sump in response to an occurrence of at least one of anover-pressure condition and an over-temperature condition in said secondpiston chamber.
 12. The drive axle of claim 11 wherein said secondhydromechanical coupling includes a first flow path for supplyinghydraulic fluid from said sump to an inlet of said second pump, a secondflow path for supply hydraulic fluid from an outlet of said second pumpto said second piston chamber, and a third flow path through said secondpiston for venting fluid from said second piston chamber to said sump,and wherein said second control valve is located in said third flowpath.
 13. The drive axle of claim 11 wherein said second control valveincludes a valve housing defining a valve chamber in fluid communicationwith said sump and a flow port providing fluid communication betweensaid second piston chamber and said valve chamber, and a thermal unloadvalve having a thermal actuator mounted in said valve chamber and avalve member movable from a first position to a second position when thetemperature of the fluid in said valve chamber exceeds a predeterminedtemperature value, said valve member is operable in its first positionto prevent fluid flow through said flow port and is further operable inits second position to permit fluid flow through said flow port forventing said second piston chamber.
 14. The drive axle of claim 11wherein said second control valve includes a pressure relief valve forventing fluid from said second piston chamber to said sump when thefluid pressure in said second piston chamber exceeds a predeterminedpressure value.
 15. The drive axle of claim 11 wherein said secondcontrol valve further includes a flow regulator for regulating flow ofhydraulic fluid from said second piston chamber to said sump to controlthe fluid pressure in said second piston chamber for actuating saidfirst piston.
 16. The drive axle of claim 11 wherein said second pistonincludes a radial web segment sealed relative to said second pistonchamber, a rim segment engageable with said second transfer clutch, anda cup segment formed between said web segment and said rim segment, saidsecond control valve is mounted in said cup segment.
 17. A drive axleassembly for use in a motor vehicle to transfer drive torque from apowertrain to a pair of wheels, comprising: a first hydromechanicalcoupling having an input shaft driven by the powertrain, a pinion shaft,a first transfer clutch opearbly disposed between said input shaft andsaid pinion shaft, a first piston disposed in a first piston chamber andactuatable to engage said first transfer clutch for transferring drivetorque to said pinion shaft, a first fluid pump for pumping hydraulicfluid from a sump to said first piston chamber in response to relativerotation between said input shaft and said pinion shaft, and a controlvalve mounted to said first piston, said control valve operable to ventfluid from said first piston chamber to said sump in response to theoccurrence of either of an over-pressure and an over-temperaturecondition; first and second output shafts adapted for connection to thepair of wheels; a casing driven by said pinion shaft; a secondhydromechanical coupling having a second transfer clutch operablydisposed between said casing and first output shaft, a second pistondisposed in a second piston chamber which is actuatable in response tofluid pressure in said second piston chamber to engage said secondtransfer clutch, and a second pump for pumping fluid from said sump tosaid second piston chamber in response to relative rotation between saidcasing and said first output shafts; and a third hydromechanicalcoupling having a third transfer clutch operably disposed between saidcasing and said second output shaft, a third piston disposed in a thirdpiston chamber which is actuatable in response to fluid pressure in saidthird piston chamber to engage said third clutch, and a third pump forpumping fluid from said sump to said third piston chamber in response torelative rotation between said casing and said second output shaft. 18.The drive axle of claim 17 wherein said first hydromechanical couplingincludes a first flow path for supplying hydraulic fluid from said sumpto an inlet of said first pump, a second flow path for supply hydraulicfluid from an outlet of said first pump to said first piston chamber,and a third flow path through said first piston for venting fluid fromsaid first piston chamber to said sump, and wherein said control valveis located in said third flow path.
 19. The drive axle of claim 17wherein said control valve includes a valve housing defining a valvechamber in fluid communication with said sump and a flow port providingfluid communication between said first piston chamber and said valvechamber, and a thermal unload valve having a thermal actuator mounted insaid valve chamber and a valve member movable from a first position to asecond position when the temperature of the fluid in said valve chamberexceeds a predetermined temperature value, said valve member is operablein its second position to permit fluid flow through said flow port forventing said first piston chamber.
 20. The drive axle of claim 17wherein said first piston includes a radial web segment sealed relativeto said first piston chamber, a rim segment engageable with said firsttransfer clutch, and a cup segment formed between said web segment andsaid rim segment, said control valve being mounted in said cup segment.21. The drive axle of claim 20 wherein said control valve comprises: atubular housing retained in said cup segment of said first piston anddefining an inlet port communicating with said first piston chamber, afirst chamber communicating with said inlet port, a second chambercommunicating with said sump, and a flow aperture between said first andsecond chambers; and a valve member moveable from a first positionclosing said flow aperture when the fluid pressure in said first chamberis less than a predetermined pressure value to a second position foropening said flow aperture when the fluid pressure in said first chamberexceeds said predetermined pressure value.
 22. The drive axle of claim21 wherein said control valve further includes a thermal actuatoroperable to move said valve member from its first position to its secondposition when the fluid temperature in said first chamber exceeds apredetermined temperature value.
 22. The drive axle of claim 17 whereinsaid second hydromechanical coupling further includes a second controlvalve that is operable to vent fluid from said second piston chamber tosaid sump in response to an occurrence of at least one of anover-pressure condition and an over-temperature condition in said secondpiston chamber.
 23. The drive axle of claim 22 wherein said thirdhydromechanical coupling further includes a third control valve that isoperable to vent fluid from said third piston chamber to said sump inresponse to an occurrence of at least one of an over-pressure conditionand an over-temperature condition in said second piston chamber.
 24. Adrive axle for use in a motor vehicle to transfer drive torque from apowertrain to a pair of wheels, comprising: a housing; an input shaftrotatably supported within said housing and adapted to be driven by thepowertrain; an interaxle portion supported by said housing and driven bysaid input shaft, comprising a first transfer clutch operablyinterconnecting said input shaft and a pinion shaft, a first clutchactuator comprising a first piston selectively operable to engage saidfirst transfer clutch, a first pump for selectively pumping actuationfluid to said first piston for actuation thereof; and a differentialportion supported within said housing, comprising a drive case rotatablydriven by said pinion shaft, first and second output shafts driven bysaid drive case adapted for connection to the wheels, a second transferclutch operably disposed between said drive case and said first outputshaft, a second clutch actuator operable to engage said second transferclutch for limiting relative rotation between said drive case and saidfirst output shaft, and a second pump for selectively pumping actuationfluid to said second clutch actuator for engaging said second clutch.25. The drive axle of claim 24 wherein said first clutch actuatorfurther comprises a first control valve for selectively disengaging saidfirst transfer clutch in response to a first condition.
 26. The driveaxle of claim 25 wherein said first control valve comprises a pressurerelief valve biased to a first position and wherein said first conditionis an over-pressure condition and once achieved said pressure reliefvalve is biased to a second position for relieving said over-pressurecondition.
 27. The drive axle of claim 25 wherein said first controlvalve selectively disengages said first clutch in response to a secondcondition.
 28. The drive axle of claim 27 wherein said first controlvalve further comprises a thermal actuator sensing said second conditionfor selectively actuating said relief valve to a second position,wherein and said second condition is an over-temperature condition andonce sensed said thermal actuator actuates said relief valve to saidsecond position for enabling cooling of said first transfer clutch. 29.The drive axle of claim 24 wherein said second pump is operably disposedbetween said drive case and said first output shaft, whereby arotational speed differential therebetween initiates pumping action ofsaid second pump for actuating said second clutch actuator for engagingsaid second clutch.
 30. The drive axle of claim 24, wherein saiddifferential portion further comprises: a third clutch in operablecommunication with both said drive case and said second output shaft; athird clutch actuator selectively operable to engage said third clutchfor prohibiting relative rotation between said drive case and saidsecond output shaft; and a third pump for selectively pumping actuationfluid to said third clutch actuator for engaging said third clutch. 31.The drive axle of claim 30 wherein said differential portion furthercomprises second and third control valves, respectively associated withsaid second and third clutch actuators, each for selectively disengagingsaid second and third clutches, respectively, in response to a firstcondition.
 32. The drive axle of claim 31 wherein said second and thirdcontrol valves each comprise a relief valve biased in a first positionand wherein said first condition is an over-pressure condition and onceachieved said relief valve is biased to a second position for relievingsaid over-pressure condition.
 33. The drive axle of claim 31 whereinsaid second and third control valves selectively disengage said secondand third clutches, respectively, in response to a second condition. 34.The drive axle of claim 33 wherein said second and third control valveseach further comprise a thermal actuator sensing said second conditionfor selectively actuating said relief valve to a second position,wherein said second condition is an over-temperature condition and oncesensed said thermal actuator actuates said relief valve to said secondposition for enabling cooling.
 35. The drive axle of claim 30 whereinsaid third pump is operably disposed between said drive case and saidsecond output shaft, whereby a rotational speed differentialtherebetween initiates pumping action of said third pump for actuatingsaid third clutch actuator for engaging said third clutch.